74 JoUEIS^AT. OF THE MiTClIELL SoCIETY [Scptoilher 



Still Doelter (Monatsli. Cliein. 31: oVJ) concludes from his own 

 investigations that Spezia is wrong in assuming that iron confers the 

 color and believes that it is due to a colloidal substance of unknown 

 nature. With regard to density he finds the green zircon to have the 

 lowest and the colorless to have the highest (4.74). Green and yel- 

 low-green zircon, he thinks have a different coloring matter from 

 brown and red (hyacinths). Stevanovic (Z, Kryst. 37: 247) states 

 that the bi-axial green zircon (D. 4.3) changes on heating into the 

 uniaxial normal with a density of 4.7. 



The varying accounts as to the properties and behavior of zircons 

 may be in part explained by the somewhat wide variations in the 

 composition of this mineral coming from different localities and the 

 neglect on the part of earlier investigators to exclude the infiltrations 

 of foreign matter which necessarily vitiate their results. Due pre- 

 caution was exercised by Doelter and other recent workers to remove 

 as far as possible such material as did not form a component part of 

 the crystals. 



Further light has been thrown on these changes in zircons by a 

 study of their radio activity and the action of radium emanations 

 upon them. 



The radio-activity of zircons is markedly greater than that of any 

 other hard mineral occurring in igneous rocks. Further, zircons 

 contain hundreds of times more helium than the average rock of which 

 they are constituent parts, and Strutt (Proc. Roy. Soc. A. 78, 152: 

 A. 83, 298: A, 89, 405) has made use of this fact as a means of de- 

 termining the geologic age of the surrounding rock. This radio- 

 activity is in excess of the uranium or thorium contents and indicates 

 the presence of an accumulation of radium. The uranium-lead 

 ratio has been determined by Holmes, the percentage of uranium 

 found being 0.0019, and of lead 0.000085 (Proc. Roy. Soc. A. 85, 

 248). Zircons show a greater radio-activity, also, than any other 

 mineral associated with, monazite. 



The fact that the only mineral known to contain argon is the zircon 

 mineral, malacone, has also aroused connnent, but not enough investi- 

 gation for complete confirmation (Kitchin and Masterson, Loud. 



